The present invention relates to a radio frequency powered semiconductor device using a surface-mounting resin-molded package and to a lead frame for the same.
Conventionally, a radio frequency semiconductor device was often assembled using a ceramic package. In recent years, however, a plastic package, molded with a molding resin material, is used more often than a ceramic package in order to reduce costs.
Hereinafter, a conventional method for fabricating a radio frequency powered semiconductor device using a lead frame for a plastic package will be described with reference to FIGS. 9(a) through 9(c).
FIG. 9(a) illustrates a planar structure during a bonding process step in the conventional method for fabricating a radio frequency powered semiconductor device. As shown in FIG. 9(a), a semiconductor chip 101 incorporating a radio frequency integrated circuit is adhered to the center portion of a square die pad 102 with a silver paste member 103. Herein, the direction along the longer sides of the die pad 102 is assumed to be X-axis direction and the direction along the shorter sides of the die pad 102 is assumed to be axis direction. A pair of grounding leads 104 are connected to both ends of the die pad 102 in the X-axis direction. A plurality of leads 105, extending in the Y-axis direction and being spaced apart from each other in the X-axis direction, are disposed to above and below the die pad 102 and to be spaced apart from the respective longer sides of the die pad 102 in the Y-axis direction. A grounding pad 101a formed on the semiconductor chip 101 for grounding the semiconductor chip 101 is electrically connected to a grounding point formed on the die pad 102 via a grounding wire 106.
FIG. 9(b) is a plan view of a semiconductor device formed by performing a step of integrally encapsulating the respective inner ends of the semiconductor chip 101, the die pad 102, the grounding leads 104 and the leads 105 with a molding resin 107 and then a bending step of molding the respective leads in predetermined shapes. FIG. 9(c) is a cross-sectional view thereof taken along the line 9C--9C of FIG. 9(b). As shown in FIG. 9(c), the grounding leads 104 are connected to a grounding land 108 such as a mounting substrate 109 and grounded. The molding resin material used as a plastic molding resin includes thermosetting epoxy resins and fillers of silica.
In the field of radio frequency powered semiconductor devices in general, it is strongly demanded to improve the heat radiation property of the molding resin 107 (i.e., to suppress the heat resistance thereof) and shorten the grounding wire 106 in order to stably operate the radio frequency powered semiconductor devices over the entire frequency regions ranging from a DC (direct current) region up to a radio frequency region.
As shown in FIG. 9(a), in the conventional radio frequency powered semiconductor device, the heat generated from the semiconductor chip 101 is conducted to the grounding land 108 (formed on the mounting substrate 109 as shown in FIG. 9(c)) through the die pad 102 and the grounding leads 104. However, in this device, the heat is conducted over a long distance from the semiconductor chip 101 to the grounding land 108 also functioning as an radiator for radiating the heat to the outside of the molding resin 107. In addition, it is difficult to secure sufficiently large grounding areas between the grounding leads 104 and the grounding land 108. Thus, this type of device cannot efficiently radiate the heat generated from the semiconductor chip 101. Accordingly, in the case of using the semiconductor chip 101 as a power converter, thermal runaway is adversely caused in the semiconductor chip 101 because the heat generated from the semiconductor chip 101 is not radiated sufficiently. As a result, the chip 101 is broken down and the reliability thereof is very low in such a case.
In a small-sized surface-mounting package, not only radiation properties but also mechanical strength thereof are problems. For example, if the thickness of the molding resin 107 is partially reduced on the bottom, on which the surface-mounting package is mounted, for the purpose of improving the radiation properties thereof, then the encapsulating strength of the molding resin 107 is disadvantageously decreased. Consequently, the leads 105 and the die pad 102 might be unintentionally detached from the molding resin 107.
Moreover, when the semiconductor chip 101 is mechanically mounted at (or die-bonded to) a predetermined position on the die pad 102 as shown in FIG. 9(a), positional misalignment of about 0.1 mm to about 0.5 mm is likely to be caused between the predetermined position and the actually mounted position of the semiconductor chip 101 on the die pad 102 in each of the X- and Y-axis directions. If such positional misalignment is caused in mounting the semiconductor chip 101, a mounting margin of about 0.5 mm to about 1 mm should be provided in the Y-axis direction for the grounding point 102a to be located with respect to the longer side of the semiconductor chip 101 when the bonding pad 101a on the semiconductor chip 101 is connected to the grounding point 102a on the die pad 102 via the grounding wire 106. Thus, the length of the grounding wire 106 becomes longer by the mounting margin. Accordingly, in the case of operating the semiconductor chip 101 as a power converter over a wide frequency region ranging from DC to radio frequency, oscillation adversely results from the parasitic inductance of the grounding wire 106 and the chip 101 cannot be operated stably any more.
Furthermore, when the bonding pad 101a is connected to the grounding point 102a via the grounding wire 106 in the same way after the semiconductor chip 101 has been adhered at the predetermined position of the die pad 102 with an adhesive such as the silver paste member 103, a certain distance should be provided between the longer side of the semiconductor chip 101 and the grounding point 102a on the die pad 102, considering the expansion of the silver paste member 103 to the periphery of the semiconductor chip 101. In such a case, the length of the grounding wire 106 also needs some margin.
Moreover, in the bending step, the grounding leads 104 and the leads 105 are appropriately bent such that the bottoms of the grounding leads 104 and the leads 105 are located at substantially the same level as that of the bottom of the molding resin 107 when the packaged semiconductor device is mounted onto the mounting substrate. In this bending step, the extended portion of any of the grounding leads 104 and the leads 105 should have a length of about 2 mm to about 15 mm from the corresponding side of the molding resin 107, from which the lead 104 or 105 extends. Thus, since none of these leads 104 and 105 can have a shortened length, the mounting area of the semiconductor device cannot be reduced.